Reduce voltage of 50V to 35V of Powersupply, regulator ? - diyAudio
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Old 30th March 2009, 09:26 PM   #1
jbdivx is offline jbdivx  France
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Talking Reduce voltage of 50V to 35V of Powersupply, regulator ?

Hi everybody , i'm sorry , i'am french

I have a toroidal transfomer 500VA 2x37V

I will go to do an amplifier with TDA7293 or LM3875, LM3886...

But i have a problem : 37V * 1.41 = 50V approximatively

So, i need to reduce voltage of about 15V for supply a LM3875 for example.

So , can i use a regulator same the LM338T for reduce the voltage ?
http://www.datasheetcatalog.org/data...a0g60wc4wy.pdf

Is that using such a regulator can degrade the signal output of the amplifier and therefore have a sound worse ?

Sorry for my poor , bad , low , English.....

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Old 30th March 2009, 10:01 PM   #2
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No need to be sorry for being French.

The following link seems to be just, what you are looking for. http://www.romanblack.com/gainamp.htm
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Old 30th March 2009, 11:01 PM   #3
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I am actually in the process of using a LT1084 (or LT1083) regulator
i will hopefully post my results as soon as i get the remaining parts in...

Dave
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Old 31st March 2009, 02:30 AM   #4
jbdivx is offline jbdivx  France
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Thanks for you help

I just saw the page, but I have a problem , the assembly is too difficult for me ( utilisation of a PNP transistor )

I juste want , if it is possible to find a transistor which can do the job

I finded the LM350 (3A max) and the LM338 (5A max) but both of them ,

when Vin - Vout = approximatively 20 V have a fall of Iout

So my idea is of take 2 x LM350 in parallel (it is already difficult for me)

So , do you think , it 's a good idea ?
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Old 31st March 2009, 04:52 AM   #5
sasmit is offline sasmit  India
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paralelling requlators by simply connecting them in paralell won't work..if thats what you're thinking .. LM338 data sheet has many schematics on how to build a high current psu with multiple regulators.
If you're dropping too much voltage across the regulator you'll not be able to achieve the maximum current capacity of the regulator. If you're just gonna run 2*LM3886 .. 1 LM338 ( one per rail ) will be upto the job...but might run a bit warmer than usual. As regards sound quality there won't be muh difference, if your supply is adequately sized.
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Old 31st March 2009, 10:37 AM   #6
Ted205 is offline Ted205  United Kingdom
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is another transformer out of the question ?
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Old 31st March 2009, 03:35 PM   #7
jbdivx is offline jbdivx  France
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I don't have other transfomer .....

sasmit, I want to use LM350 - 3-Amp Adjustable Regulator

http://www.national.com/mpf/LM/LM350.html

But , 2X LM350 for each canal ! in parralel....

Moreover , if i use a regulator , i must do this installation :

SoftStart => Transformer toroidal => Diode Bridge

=> Capacitor (1x15000uF) => Regulator => Capacitor (1x15000uF)

=> Capacitor MKP


I have only capacitor of 15000uF 63v at house ( I have 8 capacitors )

30 000uF for each canal for a TDA or a LM3875 is enough sufficient ?
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Old 1st April 2009, 09:32 AM   #8
sasmit is offline sasmit  India
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you do not need a very large cap after the requlator a 100 ~ 470uf shoud be there on the output of the regulator board ( close to the regulator output pins )and 2*1000uf ( one per rail ) on the amp board..( a small value 10uf or so and a film cap close to chip pins should already be there on the pcb )

Be sure to use the protection diodes for the regulators mentioned in the application note

on the 15000uf before the regulator solder a 0.1uf ( ceramic preferably ) directly onto the cap legs.

30000uf should be sufficient for both your channels ..

The MKP you mentioned at the end might already be present in form of a small bypass cap near the chip pins..( it should be a ceramic or tantalum rahter than film )

for bypassing powersupply a multilayer ceramic or even a disc type will perform better than MKP


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Old 1st April 2009, 09:39 AM   #9
sasmit is offline sasmit  India
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if you're going to use 2*LM3886 then 2 LM350's should just about do it, one on each rail i.e. one on +ve another on - ve.

If you're planning on paralelling you'll need 4 LM350... 2 per each rail and additional components like an opamp that will be used for paralelling the regulators.
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Old 1st April 2009, 10:56 AM   #10
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You could always remove some of the secondary windings from the transformer. It might prove to be a huge ordeal, but will lower the voltage to what you need. The good thing is that you have a toroid type. These are much easier to unstack.
Tad
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